The present invention relates to a method of inspecting the external appearance and reviewing defects of a semiconductor, or in particular to a technique for increasing the speed of an apparatus for reviewing defects of a semiconductor.
The conventional method of inspecting and reviewing defects of a semiconductor generally employs a comparative inspection in which the image of an inspected portion is compared with the image of the corresponding portion of a conforming article, and the difference between them is extracted as a defect. The method in which the images of the external appearance of the portions of the same design of different chips in the same wafer are compared for inspection is called a chip comparison method, while the method in which the images of the external appearance of the portions designed to have the same external appearance in the same chip are compared for inspection utilizing the characteristics of the areas having a periodic pattern such as a memory cell is called a cell comparison method. In the inspection areas adapted for the cell comparison, the semiconductor pattern can be considered to have a predictable periodic pattern. The first category of the cell comparison method is disclosed in JP-A-2000-67243, in which a single periodic pattern is stored in advance as a reference image and compared with a plurality of images of a plurality of inspected portions thereby to extract a defective area. In the second category of the cell comparison method, as disclosed in JP-A-2000-195458, a periodic pattern is divided into a plurality of rectangular areas, each of which is displaced by an integer multiple of the pattern period, and it is determined that a defect exists in a rectangular area associated with the maximum total sum of the differences.
The first category of the cell comparison method presupposes that the periodic pattern is identical at each position of the wafer. It is therefore difficult to successfully meet the situation where a plurality of different periodic patterns exist in a wafer. Also, with regard to the first category of the cell comparison method, in spite of the description about the mode for automatically determining whether the pattern at an inspection position is periodic or not, it is difficult to determine automatically only from an image whether a given pattern is periodic or not. Further, the process for automatic determination of a periodic pattern is accompanied by a great amount of calculations, and the first category of the method requires a waiting time before this determination process is completed. Furthermore, the second category of the method, which presupposes that the pattern at the inspection position is periodic, is not applicable in the case where the pattern periodicity is unknown.